Such a packetizer arrangement is already known in the art, e.g. from the article `Application of the multipath self-routing switch in a combined STM/ATM cross-connect system`, by B. Pauwels et al., International Switching Symposium, October 1992, Yokohama, Vol. 1, pp. 324-328. Therein, the incoming data stream is constituted by a Synchronous Digital Hierarchy (SDH) signal according to a STM-N (Synchronous Transport Module-N) frame wherein VC-n (Virtual Containers-n) are mapped, the individual data streams being mapped into these VC-n. The packet streams are applied to a packet switch and all data portions of a same individual data stream, i.e. all packets of a same packet stream, are switched to a same output of this packet switch. This packet switch is a so-called multipath self-routing (MPSR) switch wherein the paths followed by different packets of a same packet stream may be different (multi-path), and wherein a packet is routed from an input to an output of the packet switch on the basis of a self-routing tag included in this packet. In order to switch the individual data streams or VC-n from inputs to outputs of the packet switch, the VC-n are packetized into multi slot cells (MSC) constituting the packets of the packet streams. Each packet includes a first slot including a self-routing tag indicating to which packet switch output the packet is to be switched, and further slots comprising a number of consecutive bytes belonging to an individual data stream, these bytes constituting the set of subsequent data portions.
Due to the way in which the individual data streams (VC-n) are multiplexed in the incoming data stream (STM-N frame), a plurality of packets may become ready for being forwarded to the packet switch at nearly the same time, i.e. with intervals of only one byte. Thereby, when the number of individual data streams multiplexed in the incoming data stream exceeds the number of packets which can be simultaneously forwarded to the packet switch, not all packets can be forwarded to the packet switch at the time they become ready therefor so that some of them have to be buffered in an input buffer. Thereby, these packets are submitted to an additional delay which is at most about equal to the packetization delay. Indeed, in view of the fact that the maximum bit rate for data of the individual data streams output from a packetizer device obviously has to be at least equal to the bit rate at which this data is input to this packetizer device, the packet can always be forwarded to the packet switch within this maximal additional delay. Furthermore, it should be noted that subsequent packets of a packet stream are formed with intervals which are about equal to the packetization delay, so that the additional delay is not allowed to be longer than the packetization delay in order to prevent the sequence order of the packets from being lost.
On the other hand, the packet switch has a plurality of inputs to each of which packets derived by the packetizer arrangement from individual data streams are provided. Furthermore, the packet switch used has the characteristic that the switching delay between any of its inputs and any of its outputs follows a sharply peaked distribution, i.e. is substantially a constant. When different packets destined to the same packet switch output are simultaneously released by the packetizer arrangement to different packet switch inputs and are hence substantially simultaneously received by the latter same output, a contention problem occurs, i.e. the packets have to be buffered at this output. Therefore, a buffer is included in the packet switch which is dimensioned so as to be able to handle such a simultaneous arrival of a number of packets. Even with such a buffer, packet loss may occur. However, when the incoming data stream is an STM data stream, traffic patterns of the packets are persistent, i.e. the packet loss occurs on a regular basis. In order to break this persistent behaviour, randomization of the instant at which the packet is forwarded from the packetizer arrangement to the packet switch may be introduced at the inputs. This may be done by means of the above mentioned input buffer. Randomization is then most effective when the introduced randomization delay is uniformly distributed between zero and the packetization delay, i.e. when the maximum value of the above additional delay is made as large as possible. Remembering the restriction already mentioned above that this maximum value should not exceed the packetization delay, the maximal additional delay is advantageously chosen equal to the packetization delay.
Furthermore, even when the forwarding instant of the packets to the packet switch is randomized as described above, the forwarding instants of the packets are not evenly distributed in time, i.e. the packets are still forwarded in bursts to the packet switch which causes an increased packet loss ratio in the packet switch and an increased switching delay from input to output of the packet switch, both with respect to a situation wherein the packets are forwarded to the packet switch at a more evenly spread or constant rate.